The present invention relates to a process for isotopic separation by vibrational transfer observed on sulphur, but also applicable in principle to any other separation of isotopes with a varying degree of effectiveness, e.g. isotopes of hydrogen, boron, carbon, lithium, xenon, plutonium, uranium, etc.
At present, several isotopic separation processes are known. One of these processes is based on the multiphoton effect, which will briefly be described. The multiphoton effect consists of the absorption by the vibrations of a molecule of the light energy emitted by a light source by the successive absorption of several photons until the molecule is dissociated. In such an effect, the molecule is a priori isolated from the other molecules and dissociation takes place for a relatively short and relatively high energy light source, i.e. in a time less than the collision time between molecules.
This effect causes difficult interpretation problems. In particular, it is necessary to understand how photons of the same energy can be absorbed by transitions, whose energy variation decreases on climbing the vibrational levels of a molecule. The phenomen is usually described by considering that the first vibrational levels are discrete and well separated from one another and that their light absorption has a narrow frequency resonance. The higher levels are much denser forming a quasi-continuum and their light absorption has a wider resonance band.
Isotopic selectivity by the multiphoton effect has been described in a first article by R. V. AMBARTZUMIAN, V. S. LETOKOV, E. A. RAYABOV, N. V. CHEKALIN, which appeared in JEPT lett, 1974, Vol. 21, pp. 171-173, and in a second article by J. L. LYMAN, R. J. JENSEN, J. RINK, C. P. ROBINSON, S. D. ROCKWOOD which appeared in Appl. Phys. Lett., 1975, Vol. 27, pp. 87-89.
Isotopic selectivity is explained by the fact that the first energy levels of the molecule have narrow resonance band widths, which can be less than the displacement of the vibrational energy levels of the two isotopes of the molecule in question. For example, for the sulphur hexafluoride molecule of formula SF.sub.6, the isotopic shift between the isotopic varieties .sup.32 SF.sub.6 and .sup.34 SF.sub.6 is 17.5 cm.sup.-1 (948.0 cm.sup.-1 for the third mode of .sup.32 SF.sub.6 and 930.5 cm.sup.-1 for the third mode of .sup.34 SF.sub.6).
Unfortunately, the use of this multiphoton effect for the isotopes of a given atom presupposes that a molecule is available which contains this atom and is able to absorb the frequency of a power light source. In the infrared region of vibrational energy levels, only the carbon dioxide laser is able to supply intense light pulses around 940 cm.sup.-1, which considerably limits the isotopic separation possibilities. Thus, even if it is possible to separate the isotopic varieties .sup.32 SF.sub.6 and .sup.34 SF.sub.6 by the multiphoton effect by using such a light source, at present no intense light source is available for molecules having a resonant frequency below 800 cm.sup.-1.